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The Latest Research Items

Switchable Micropatterned Surfaces

Researchers in the Ashby and Sheiko groups have fabricated textured surfaces capable of reversibly changing in response to a thermal stimulus. These surfaces are fabricated with PRINT© molds provided by the DeSimone Lab, and could find use in applications requiring modular surface wetting or roughness. Reversibly switching topography on micrometer length scales greatly expands the functionality of stimuli-responsive substrates.

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In an article published in ACS Applied Materials & Interfaces the groups report the first usage of reversible shape memory for the actuation of two-way transitions between microscopically patterned substrates, resulting in corresponding modulations of the wetting properties. Reversible switching of the surface topography is achieved through partial melting and recrystallization of a semi-crystalline polyester embossed with microscopic features. This behavior is monitored with atomic force microscopy, AFM, and contact angle measurements. The groups demonstrate that the magnitude of the contact angle variations depends on the embossment pattern.

 

Fuel from Carbon Dioxide

Reduction of CO2 to fuels and chemicals has attracted considerable attention as petroleum reserves dwindle. Formic acid and formate are two-electron reduced products of CO2 and can serve as hydrogen storage materials, as precursors to methanol, as reducing agents in organic synthesis, as the fuel in fuel cell applications, as an environmentally attractive substitute for mineral acids in applications in mining, drilling, and hydrofracking, and as a feedstock for bacteria in the production of gasoline substitutes. Currently, formic acid is produced by a multi-step chemical synthesis. Developing an efficient, single-step electrocatalytic method for formate/formic acid production could be of value in reducing cost, thus enhancing their use in possible fuel cell and related applications.

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In an article published in Angewandte Chemie, researchers in the Brookhart and Meyer groups describe how an iridium pincer dihydride catalyst was immobilized on carbon nanotube-coated gas diffusion electrodes, GDEs, by using a non-covalent binding strategy. The as-prepared GDEs are efficient, selective, durable, gas permeable electrodes for electrocatalytic reduction of CO2 to formate. High turnover numbers,circa 54 000,and turnover frequencies, circa 15 s-1 were enabled by the novel electrode architecture in aqueous solutions saturated in CO2 with added HCO3-.

 

Organ-on-a-Chip

New advances enable long-term organotypic culture of colonic epithelial stem cells that develop into structures known as colonoids. Colonoids represent a primary tissue source acting as a potential starting material for development of an in vitro model of the colon. Key features of colonic crypt isolation and subsequent colonoid culture have not been systematically optimized compromising efficiency and reproducibility. Research from the Allbritton Group, published in the Journal of Biological Engineering, show how murine crypt isolation yield and quality can be optimized, and colonoid culture efficiency measured in microfabricated culture devices.

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Improved crypt isolation and 3-D colonoid culture, along with an understanding of colonic epithelial cell behavior in the presence of microfabrication substrates will support development of "organ-on-a-chip" approaches for studies using primary colonic epithelium.

 

Immobilization of Lambda Exonuclease

The process of immobilizing enzymes onto solid supports for bioreactions has some compelling advantages compared to their solution-based counterpart including the facile separation of enzyme from products, elimination of enzyme autodigestion, and increased enzyme stability and activity. Researchers in the Soper Group, published in Analytical Chemistry report the immobilization of λ-exonuclease onto poly(methylmethacrylate) (PMMA) micropillars populated within a microfluidic device for the on-chip digestion of double-stranded DNA.

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The group's results suggest that the efficiency for the catalysis of dsDNA digestion using λ-exonuclease, including its processivity and reaction rate, were higher when the enzyme was attached to a solid support compared to the free solution digestion. The results from this work will have important ramifications in new single-molecule DNA sequencing strategies that employ free mononucleotide identification.

 

Imaging Charge Separation in Nanowires

Silicon nanowires incorporating p-type/n-type (p n) junctions have been introduced as basic building blocks for future nanoscale electronic components. Controlling charge flow through these doped nanostructures is central to their function, yet our understanding of this process is inferred from measurements that average over entire structures or integrate over long times.

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Published in Nano Letters, researchers from the Cahoon and Papanikolas Groups describe how they used femtosecond pump-probe microscopy to directly image the dynamics of photogenerated charge carriers in silicon nanowires encoded with p-n junctions along the growth axis. Initially, motion is dictated by carrier-carrier interactions, resulting in diffusive spreading of the neutral electron-hole cloud. Charge separation occurs at longer times as the carrier distribution reaches the edges of the depletion region, leading to a persistent electron population in the n type region. Time-resolved visualization of the carrier dynamics yields clear, direct information on fundamental drift, diffusion, and recombination processes in these systems, providing a powerful tool for understanding and improving materials for nanotechnology.

 

Ultrafast Carrier Dynamics II

In a collaboration between the Cahoon and Papanikolas groups, published in the Journal of Physical Chemistry C, ultrafast charge carrier dynamics in silicon nanowires (NWs) grown by a vapor–liquid–solid mechanism were interrogated with optical pump–probe microscopy. The high time and spatial resolutions achieved by the experiments provide insight into the charge carrier dynamics of single nanostructures. Individual NWs were excited by a femtosecond pump pulse focused to a diffraction-limited spot, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Photoexcited carriers undergo both electron–hole recombination and diffusional migration away from the excitation spot on similar time scales. The evolution of the carrier population is monitored by a delayed probe pulse that is also focused to a diffraction-limited spot. When the pump and probe are spatially overlapped, the transient signal reflects both recombination and carrier migration. Diffusional motion is directly observed by spatially separating the pump and probe beams, enabling carriers to be generated in one location and detected in another.

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Quantitative analysis of the signals yields a statistical distribution of carrier lifetimes from a large number of individual NWs. On average, the lifetime was found to be linearly proportional to the diameter, consistent with a surface-mediated recombination mechanism. These results highlight the capability of pump–probe microscopy to quantitatively evaluate key recombination characteristics in semiconductor nanostructures, which are important for their implementation in nanotechnologies.

 

Controlling Molecular Weight

Molecular weight, MW, its distribution and dispersity, PDI, of polymers are possibly the most important characteristics that distinguish polymers from small organic molecules. Conjugated polymers are no exception to this. For polymer solar cells, a high MW is usually desirable. For example, high MW polymers have good viscosity desirable for thin films coating. More importantly, it appears that a high MW is beneficial for a higher current of solar cells. However, a number of questions had still remained to be answered, such as: Is there any appropriate MW for conjugated polymers used for solar cells? If so, can we control the MW? What about PDI?

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Published in Advanced Materials, the You Group offers some insights towards these outstanding issues concerning MW. Taking their well-acclaimed conjugated polymer, PBnDT-FTAZ, as the model system, they created a set of polymers with precisely controlled MW by adjusting stoichiometric ratio of two monomers, following the classic Carothers question. In collaboration with the Ade Group at NCSU, the You Group carefully investigated this set of PBnDT-FTAZ with different MW and discovered that the MW significantly influences the morphology and structural order of PBnDT-FTAZ in its bulk heterojunction solar cells, with highest efficiency, over 7%, resulting with use of a MW of 40 kg/mol. Additionally, by recreating a 40 kg/mol polymer with a higher PDI of 3.2 than the pristine 40 kg/mol polymer, PDI of 2.2, they showed that the dispersity,PDI, though largely neglected in the past, might play a role in affecting the device performance of polymer solar cells.

 

Protein Crowder Charge and Stability

Macromolecular crowding effects arise from steric repulsions and weak, nonspecific, chemical interactions. Steric repulsions stabilize globular proteins, but the effect of chemical interactions depends on their nature. Repulsive interactions such as those between similarly charged species should reinforce the effect of steric repulsions, increasing the equilibrium thermodynamic stability of a test protein. Attractive chemical interactions, on the other hand, counteract the effect of hard-core repulsions, decreasing stability.

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Mohona Sarkar and Joe Lu, researchers in the Pielak Group, tested these ideas, published in Biochemistry, by using the anionic proteins from Escherichia coli as crowding agents and assessing the stability of the anionic test protein chymotrypsin inhibitor 2 at pH 7.0. The anionic protein crowders destabilize the test protein despite the similarity of their net charges. Thus, weak, nonspecific, attractive interactions between proteins can overcome the charge–charge repulsion and counterbalance the stabilizing effect of steric repulsion.

 

Dual Action Antibacterial Agents

Published in Bioconjugate Chemistry, researchers in the Schoenfisch Group describe the synthesis of nitric oxide, NO, releasing quaternary ammonium, QA, functionalized generation 1, G1, and generation 4, G4, poly(amidoamine), PAMAM, dendrimers. Dendrimers were modified with QA moieties of different alkyl chain lengths, such as methyl, butyl, octyl, dodecyl, via a ring-opening reaction. The resultant secondary amines were then modified with N-diazeniumdiolate NO donors to yield NO-releasing QA-modified PAMAM dendrimers capable of spontaneous NO release.

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The bactericidal efficacy of individual, non-NO-releasing, and dual action, NO-releasing, QA-modified PAMAM dendrimers was evaluated against Gram-positive Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa bacteria. Bactericidal activity was found to be dependent on dendrimer generation, QA alkyl chain length, and bacterial Gram class for both systems. Shorter alkyl chains, such as methylQA and butylQA, demonstrated increased bactericidal activity against P. aeruginosa versus S. aureus for both generations, with NO release markedly enhancing overall killing.

 

Reducing Drug Toxicity with PRINT

The synthesis of prodrugs is a common approach to overcome drug delivery issues, including poor aqueous solubility or permeability, and to provide site-specific release. Nanotechnology can be a powerful tool to improve drug delivery, but does so by altering the biodistribution of the encapsulated small molecule. In a report published in NanoLetters, researchers in the DeSimone Group, in collaboration with a number of Centers, Institutes, and Departments here at UNC, combined the merits of both approaches to improve the pharmacokinetics and toxicity of the chemotherapeutic docetaxel by passively targeting an encapsulated docetaxel prodrug to solid tumors, where it could selectively release and convert to active docetaxel.

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The Group used PRINT technology, Particle Replication in Nonwetting Templates, to prepare nanoparticles to passively target solid tumors in an A549 subcutaneous xenograft model. An acid labile prodrug was delivered to minimize systemic free docetaxel concentrations and improve tolerability without compromising efficacy.

 

Electrocatalytic Carbon Dioxide Reduction

Accumulation of carbon dioxide in the atmosphere is considered a major contributor to climate change. Once captured, CO2 is a potentially useful feedstock if it can be converted into formate/formic acid, carbon monoxide, or more highly reduced hydrocarbon products. Electrochemical and photoelectrochemical CO2 reduction could become an integral part of an energy storage strategy with solar- or wind-generated electricity used to store energy in the chemical bonds of carbon-based fuels.

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The Meyer Group, in collaboration with the Department of Electrical and Computer Engineering at Duke University, published in JACS, reports on how Nitrogen-doped carbon nanotubes are selective and robust electrocatalysts for CO2 reduction to formate in aqueous media without the use of a metal catalyst. An overlayer of polyethylenimine (PEI) functions as a cocatalyst by significantly reducing catalytic overpotential and increasing current density and efficiency.

 

Trifluormethyl Pyrrolidines

Researchers in the Johnson Group, published in Organic Letters, describe the stereoselective synthesis of trisubstituted 2-trifluoromethyl pyrrolidines by asymmetric Michael addition/hydrogenative cyclization.

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The direct organocatalytic addition of 1,1,1-trifluoromethylketones to nitroolefins proceeds under mild reaction conditions and low catalyst loadings to provide Michael adducts in high yield with excellent diastereo- and enantioselectivity. Catalytic hydrogenation of the Michael adducts stereoselectively generates 2-trifluoromethylated pyrrolidines bearing three contiguous stereocenters. The group members also describe a stereospecific route to epimeric 2-trifluoromethyl pyrrolidines from a common intermediate.

 

Ultrafast Carrier Dynamics

Scientists in the Cahoon and Papanikolas groups, as published in the Journal of Physical Chemistry C, have studied ultrafast carrier dynamics in silicon nanowires with average diameters of 40, 50, 60, and 100 nm using transient absorption spectroscopy. After 388 nm photoexcitation near the direct band gap of silicon, broadband spectra from 400 to 800 nm were collected between 200 fs and 1.3 ns. The transient spectra exhibited both absorptive and bleach features that evolved on multiple time scales, reflecting contributions from carrier thermalization and recombination as well as transient shifts of the ground-state absorption spectrum. The initially formed “hot” carriers relaxed to the band edge within the first 300 fs, followed by recombination over several hundreds of picoseconds.

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The charge carrier lifetime progressively decreased with decreasing diameter, a result consistent with a surface-mediated recombination process. Recombination dynamics were quantitatively modeled using the diameter distribution measured from each sample, and this analysis yielded a consistent surface recombination velocity of 2 × 104 cm/s across all samples. The results indicate that transient absorption spectroscopy, which interrogates thousands of individual nanostructures simultaneously, can be an accurate probe of material parameters in inhomogeneous semiconductor samples when geometrical differences within the ensemble are properly analyzed.

 

Chemical Vapor Deposition

Researchers in the Ramsey Group, published in Analytical Chemistry, describe a chemical vapor deposition, CVD, method for the surface modification of glass microfluidic devices designed to perform electrophoretic separations of cationic species. The microfluidic channel surfaces were modified using aminopropyl silane reagents. Coating homogeneity was inferred by precise measurement of the separation efficiency and electroosmotic mobility for multiple microfluidic devices.

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Microfluidic devices with a 23 cm long, serpentine electrophoretic separation channel and integrated nanoelectrospray ionization emitter were CVD coated with (3-aminopropyl)di-isopropylethoxysilane, APDIPES, and used for capillary electrophoresis (CE)-electrospray ionization (ESI)-mass spectrometry (MS) of peptides and proteins. Peptide separations were fast and highly efficient, yielding theoretical plate counts over 600,000 and a peak capacity of 64 in less than 90 s. Intact protein separations using these devices yielded Gaussian peak profiles with separation efficiencies between 100,000 and 400,000 theoretical plates.

 

Capturing Circulating Tumor Cells

Circulating tumor cells, CTCs, are important biomarkers of cancer progression and metastatic potential. The rarity of CTCs in peripheral blood has driven the development of technologies to isolate these tumor cells with high specificity. However, there are limited techniques available for isolating target CTCs following enumeration. Researchers in the Allbritton Group, published in Biosensors and Bioelectronics, describe a strategy to capture and isolate viable tumor cells from whole blood using an array of releasable microstructures termed micropallets.

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Clinical utility of their technology was shown through the capture, isolation and successful culture of CTCs from the blood of mice engrafted with primary human pancreatic tumors. Direct capture and isolation of living tumor cells from blood followed by analysis or culture will be a valuable tool for cancer cell characterization.

 

Strategies for Protein NMR

In-cell NMR spectroscopy, one of the pioneers of which is the Pielak Group here at Carolina Chemistry, provides insight into protein conformation, dynamics, and function at atomic resolution in living cells. Systematic evaluation of isotopic-labeling strategies is necessary to observe the target protein in the sea of other molecules in the cell.

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In a collaboration with scientists from the Chinese Academy of Sciences, published in Biochemistry, researchers in the Pielak Group investigate the detectability, sensitivity, and resolution of in-cell NMR spectra of the globular proteins GB1, ubiquitin, calmodulin, and bcl-xl-cutloop, resulting from uniform 15N enrichment, with and without deuteration, selective 15N-Leu enrichment, 13C-methyl enrichment of isoleucine, leucine, valine, and alanine, fractional 13C enrichment, and 19F labeling. Most of the target proteins can be observed by 19F labeling and 13C enrichment with direct detection because selectively labeling suppresses background signals and because deuteration improves in-cell spectra. The group's results demonstrate that the detectability of proteins is determined by weak interactions with intercellular components and that choosing appropriate labeling strategies is critical for the success of in-cell protein NMR studies.